1. Technical Field
The invention is related to a plasma reactor for processing a semiconductor substrate or wafer, and in particular to a plasma reactor in which a process material, such as a passivation additive, is sputtered from a target surface within the reactor chamber.
2. Background Art
Plasma reactors for etching polysilicon thin films from a semiconductor wafer can employ halogen gases, for example, as a main etchant in the plasma. One problem is that such gases also tend to etch any silicon dioxide thin films on the wafer that may be exposed to the plasma, so that etch selectivity may be poor. It is known that this problem may be at least ameliorated if not completely solved by introducing into the plasma a passivation additive, such as an oxygen source like silicon dioxide, to reduce the etch rate of any silicon dioxide thin film on the wafer, without reducing the etch rate of the polysilicon thin film.
An additive material such as silicon dioxide may be introduced into the plasma by sputtering of a target surface containing the additive material (e.g., quartz) to be introduced into the plasma. It has recently been suggested that the sputter target surface may be located on or near the ceiling of the reactor chamber, and that the sputtering may be driven by an RF-excited electrode placed directly over the sputter target surface. The RF-excited electrode over the sputter target surface may be located inside the chamber or outside the chamber, but in either case must be relatively close to the sputter target surface. Thus, for example, the sputter target surface itself may be the chamber ceiling, while the RF-excited electrode may be located directly over the ceiling and therefore be outside of the chamber. Typically, the wafer is located directly below the ceiling on a wafer pedestal near the floor of the chamber. By varying the RF power applied to the RF-excited electrode, the silicon dioxide deposition rate on a wafer can be controllably varied within a range between zero and seventy Angstroms per minute.
One problem posed by having an RF-excited electrode over the sputter target surface on or near the ceiling is that the RF-excited electrode will cause a significant amount of capacitively coupled current to flow either to the chamber walls or down to the wafer near the chamber floor. Current flowing to the chamber walls will increase undesirable sputtering of the chamber walls. Current flowing to the wafer or wafer pedestal will increase ion bombardment damage of microelectronic devices on the wafer. A significant amount of RF power must be applied to the RF-excited electrode in order to produce sputtering of the target surface, so that the problem cannot be solved merely by reducing the RF power applied to the electrode.
An object of the present invention is to reduce or eliminate capacitively coupled currents to the wafer or wafer pedestal induced by the RF-excited electrode over the target surface. A related object of the invention is to reduce the range or depth of capacitive coupling from the RF-excited electrode into the reactor chamber so as to reduce the effect of the electrode on the plasma near the wafer and/or near the chamber walls.